Biobased, biodegradable polymers such as polyhyroxyalkanoates (PHA's), have been produced in a variety of biomass systems such as plant biomass, microbial biomass (e.g., bacteria including cyanobacteria, yeast, fungi) and algal biomass. Genetically-modified biomass systems have recently been developed which produce a wide range of biodegradable PHA polymers and copolymers in high yield (Lee (1996), Biotechnology & Bioengineering, 49: 1-14; Braunegg et. al. (1998), J. Biotechnology, 65:127-161; Madison and Huisman (1999), Metabolic Engineering of Poly-3-Hydroxyalkanoates; From DNA to Plastic, in Microbiol. Mol. Biol. Rev., 63:21-53). The PHA's produced from biomass systems are made using renewable feedstocks, are of high molecular weight and are hydrolytically stable but can biodegrade in a number of microbial environments including soil, marine and home or industrial composters.
PHA's can be thermally processed into products in much the same way as petroleum-based thermoplastic polymer materials. Applications for PHA polymers include mulch films, nonwoven fibers, extruded foams, injection molded utensils and thermoformed trays to name a few. One industrially important processing route for PHA materials is the conversion of the polymer into an aqueous PHA emulsion or dispersion (also referred to as a latex) where the PHA is present as an amorphous colloidal particle (median diameter of 0.01-1 μm) suspended in water. Commercial applications for PHA emulsions include paints, architectural coatings, adhesives, wood lacquers, paper coatings as well as binders for time-released agricultural chemicals.
Several published patents describe processes for making emulsions from biodegradable polymers. U.S. Pat. No. 6,103,858 (BASF) describes a process for forming a latex from poly(butylene-adipate-terephthalate) which is a petroleum-based biodegradable polyester. The process, however, includes the added steps of first increasing the molecular weight of the polymer by reacting it with chain extenders, such as diisocyanates, then dissolving the polymer into an organic solvent followed by dispersion in water and finally removal of the organic solvent by vacuum distillation. The patent makes no reference to a process of making a polyester emulsion in the absence of organic solvent addition. One would further expect that the resultant emulsion would have residual amounts of solvent present in the final latex and hence contribute to a VOC (volatile organic compound) issue in practical use. U.S. Pat. No. 6,716,911 (Showa High Polymer Co. Ltd) describes a process for producing latexes from petroleum-based biodegradable polyesters (poly(butylene-succinate), poly(butylene-succinate-adipate)). A twin-screw extruder is used to melt process the polyesters with a 1% aqueous surfactant solution to give a latex dispersion. In this process, no organic solvents are used, however the patent describes the need for having a viscosity ratio between the molten polyester and the surfactant solution of <150. This indicates that the polyesters are of fairly low molecular weight and therefore have low melt temperatures and tensile strengths that limit the final properties of films produced from the latex. U.S. Pat. No. 6,103,858 (Metabolix) describes a process for producing latexes from biobased, biodegradable polyhydroxyalkanoate (PHA) polyesters. This is a multi-step process where the first step involves forming a PHA suspension from biomass containing the PHA using an aqueous recovery process involving cell digestion, washing with surfactant/peroxide followed by microfluidization, centrifugation and re-suspension of the PHA particles in water. The final step to produce an amorphous PHA latex involved heating the suspension under pressure to 190° C.-200° C. (25° C. above melt temperature of PHA) followed by rapid cooling. While the process was shown to produce aqueous PHA emulsions (latexes) with acceptable properties on a small scale, it is cost prohibitive to scale up due to the complexity of the steps required to generate the latex as well as the need for high pressure processing equipment.
The processes described above for producing latexes from biodegradable polyesters all require the polymers to be processed substantially above the melting point of the specific polyester before being combined with water or a solvent diluents before subsequent dispersion to form a latex product with an acceptably small particle size. The introduction of water as the solvent well above the boiling point of the liquid also requires high pressure containment equipment and substantial safety controls which are not typical for most latex processing equipment designs.
There is a need, therefore, to develop a simplified, more cost effective and environmentally friendly process for making biobased, biodegradable latexes from renewable resources.